1. Field of the Invention
The present invention relates to an energy storage device cell containing a combined configuration of a lithium ion battery and a lithium ion capacitor.
2. Description of the Related Art
Examples of an energy storage device cell include an electric double layer capacitor, a lithium ion battery (LIB), and a lithium ion capacitor (LIC). The electric double layer capacitor (also referred to simply as capacitor, supercapacitor, electrochemical capacitor, etc.) includes polarized electrodes (cathode and anode) opposed to each other with a separator interposed therebetween and utilizes a capacitance of an electric double layer formed on the surface of the polarized electrodes in an electrolyte solution. The lithium ion battery has a feature of being capable of charging and storing lithium in a carbon anode stably, and an oxide containing cobalt, nickel, manganese, or the like is used as a cathode.
The lithium ion capacitor has been developed as a hybrid type of the electric double layer capacitor and the lithium ion battery. The lithium ion capacitor includes a cathode of the electric double layer capacitor and an anode of the lithium ion battery. The lithium ion capacitor may obtain a voltage higher than that of the electric double layer capacitor, but has a drawback in that it is difficult to set a lower limit voltage to 0 V.
The electric double layer capacitor does not have an instantaneous power comparable to that of an aluminum electrolytic capacitor. However, the electric double layer capacitor has an advantage in that the power density of energy is large and charging and discharging can be performed in a short period of time. On the other hand, of all the energy storage device cells, the lithium ion battery has overwhelmingly high energy density, namely sustainability. If an energy storage device cell having both the instantaneous power of the electric double layer capacitor and the sustainability of the lithium ion battery can be realized, an energy storage device cell can be used for various applications, such as brake regeneration of a hybrid automobile.
Conventionally, as an energy storage device cell containing a combined configuration of a lithium ion battery and a lithium ion capacitor, there is known an energy storage device cell in which a lithium ion capacitor cathode, a common anode, and a lithium ion battery cathode are stacked in this order, and separators are provided respectively between the lithium ion capacitor cathode and the common electrode and between the common anode and the lithium ion battery cathode.
The common anode includes an anode collector foil having through-holes formed therein and an anode electrode layer applied to one surface of the anode collector foil. The anode collector layer is placed between the lithium ion capacitor cathode and the anode collector foil. The lithium ion capacitor cathode includes a capacitor cathode collector foil and a capacitor cathode electrode layer applied to one surface of the capacitor cathode collector foil. The capacitor cathode electrode layer is placed between the common anode and the capacitor cathode collector foil. The lithium ion battery cathode includes a battery cathode collector foil and a battery cathode electrode layer applied to one surface of the battery cathode collector foil. The battery cathode electrode layer is placed between the common anode and the battery cathode collector foil (see, for example, Japanese Patent Application Laid-open No. 2009-141181).
In the conventional energy storage device cell, both the sustainability of the lithium ion battery and the instantaneous power of the lithium ion capacitor can be utilized. Further, after the lithium ion capacitor portion once receives a current during rapid charging, the current flows through the lithium ion battery portion. Therefore, the lithium ion battery portion during rapid charging can be prevented from being degraded, which can prolong the cycle life of the lithium ion battery portion.
However, the area of the capacitor cathode electrode layer is the same as that of the battery cathode electrode layer, and hence a region occupied by the lithium ion battery portion in the entire energy storage device cell is small. Thus, there has been a problem that the energy density of the energy storage device cell is low.